The Kingdom of Italy (Italian: Regno d'Italia) was a state that existed from 1861when King Victor Emmanuel II of Sardinia was proclaimed King of Italyuntil 1946, when civil discontent led an institutional referendum to abandon the monarchy and form the modern Italian Republic. The state was founded as a result of the Risorgimento under the influence of the Savoy-led Kingdom of Sardinia, which can be considered its legal predecessor state.

Italy declared war on Austria in alliance with Prussia in 1866 and received the region of Veneto following their victory. Italian troops entered Rome in 1870, thereby ending more than one thousand years of Papal temporal power. Italy entered into a Triple Alliance with the German Empire and the Austro-Hungarian Empire in 1882, following strong disagreements with France about their respective colonial expansions. However, even if relations with Berlin became very friendly, the alliance with Vienna remained purely formal as the Italians were keen to acquire Trentino and Trieste, corners of Austria-Hungary populated by Italians. So during World War I, Italy accepted the British invitation to join the Allied Powers, as the western powers promised territorial compensation (at the expense of Austria-Hungary) for participation that was more generous than Vienna's offer in exchange for Italian neutrality. Victory in the war gave Italy a permanent seat in the Council of the League of Nations.

"Fascist Italy" is the era of National Fascist Party government from 1922 to 1943 with Benito Mussolini as head of government. The fascists imposed totalitarian rule and crushed the political and intellectual opposition, while promoting economic modernization, traditional social values and a rapprochement with the Roman Catholic Church. According to Payne (1996), "[the] Fascist government passed through several relatively distinct phases". The first phase (19231925) was nominally a continuation of the parliamentary system, albeit with a "legally-organized executive dictatorship". Then came the second phase, "the construction of the Fascist dictatorship proper, from 1925 to 1929". The third phase, with less activism, was 1929 to 1934. The fourth phase, 19351940, was characterized by an aggressive foreign policy: war against Ethiopia, launched from Italian Eritrea and Italian Somaliland, which resulted in its annexation; confrontations with the League of Nations, leading to sanctions; growing economic autarky; and the signing of the Pact of Steel. The war itself (19401943) was the fifth phase with its disasters and defeats, while the rump Sal Government under German control was the final stage (19431945).Fascist Italy was a leading member of the Axis powers in World War II. By 1943, the German-Italian defeat on multiple fronts and the subsequent Allied landings in Sicily led to the fall of the Fascist regime, and Mussolini was placed under arrest by order of the King Victor Emmanuel III. The new government signed an armistice with the Allies on September 1943. German forces occupied northern and central Italy, setting up the Italian Social Republic, a collaborationist puppet state still led by Mussolini and his Fascist loyalists. As a consequence, the country descended into civil war, with the Italian Co-belligerent Army and the resistance movement contending with the Social Republic's forces and its German allies. Shortly after the war and the liberation of the country, civil discontent led to the institutional referendum on whether Italy would remain a monarchy or become a republic. Italians decided to abandon the monarchy and form the Italian Republic, the present-day Italian state.

World War II or the Second World War, often abbreviated as WWII or WW2, was a global war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing military alliances: the Allies and the Axis powers. In a total war directly involving more than 100 million personnel from more than 30 countries, the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. Aircraft played a major role in the conflict, enabling the strategic bombing of population centres and the only two uses of nuclear weapons in war. World War II was by far the deadliest conflict in human history; it resulted in 70 to 85 million fatalities, a majority being civilians. Tens of millions of people died due to genocides (including the Holocaust), starvation, massacres, and disease. In the wake of the Axis defeat, Germany and Japan were occupied, and war crimes tribunals were conducted against German and Japanese leaders.

The exact causes of World War II are debated, but contributing factors included the Second Italo-Ethiopian War, the Spanish Civil War, the Second Sino-Japanese War, the Soviet–Japanese border conflicts and rising European tensions since World War I. World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland. The United Kingdom and France subsequently declared war on Germany on 3 September. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union had partitioned Poland and marked out their "spheres of influence" across Finland, Estonia, Latvia, Lithuania and Romania. From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, and formed the Axis alliance with Italy and Japan (along with other countries later on). Following the onset of campaigns in North Africa and East Africa, and the fall of France in mid-1940, the war continued primarily between the European Axis powers and the British Empire, with war in the Balkans, the aerial Battle of Britain, the Blitz of the UK, and the Battle of the Atlantic. On 22 June 1941, Germany led the European Axis powers in an invasion of the Soviet Union, opening the Eastern Front, the largest land theatre of war in history.

Japan, which aimed to dominate Asia and the Pacific, was at war with the Republic of China by 1937. In December 1941, Japan attacked American and British territories with near-simultaneous offensives against Southeast Asia and the Central Pacific, including an attack on the US fleet at Pearl Harbor which resulted in the United States declaring war against Japan. Therefore the European Axis powers declared war on the United States in solidarity. Japan soon captured much of the western Pacific, but its advances were halted in 1942 after losing the critical Battle of Midway; later, Germany and Italy were defeated in North Africa and at Stalingrad in the Soviet Union. Key setbacks in 1943—including a series of German defeats on the Eastern Front, the Allied invasions of Sicily and the Italian mainland, and Allied offensives in the Pacific—cost the Axis powers their initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned towards Germany and its allies. During 1944 and 1945, Japan suffered reversals in mainland Asia, while the Allies crippled the Japanese Navy and captured key western Pacific islands.

The war in Europe concluded with the liberation of German-occupied territories, and the invasion of Germany by the Western Allies and the Soviet Union, culminating in the fall of Berlin to Soviet troops, Hitler's suicide and the German unconditional surrender on 8 May 1945. Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender on its terms, the United States dropped the first atomic bombs on the Japanese cities of Hiroshima, on 6 August, and Nagasaki, on 9 August. Faced with an imminent invasion of the Japanese archipelago, the possibility of additional atomic bombings, and the Soviet’s declared entry into the war against Japan on the eve of invading Manchuria, Japan announced on 15 August its intention to surrender, then signed the surrender document on 2 September 1945, cementing total victory in Asia for the Allies.

World War II changed the political alignment and social structure of the globe. The United Nations (UN) was established to foster international co-operation and prevent future conflicts, with the victorious great powers—China, France, the Soviet Union, the United Kingdom, and the United States—becoming the permanent members of its Security Council. The Soviet Union and the United States emerged as rival superpowers, setting the stage for the nearly half-century-long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia. Most countries whose industries had been damaged moved towards economic recovery and expansion. Political and economic integration, especially in Europe, began as an effort to forestall future hostilities, end pre-war enmities and forge a sense of common identity.